Combined cell, also denoted shared cell or multi-sector cell in some cases, is a recent cell configuration for Long Term Evolution (LTE) and enables a multi Radio Resource Unit (RRU) deployment that does not require extra cell planning effort from a Radio Frequency (RF) perspective. This is achieved by allowing the different RRUs to use the same Physical Cell Identity (PCI) and thus all RRUs are considered, by a communication device, to be part of the same cell. The spatially separated RRUs, or group of RRUs, are called sector. A cell can contain multiple sectors, and transmission to and reception from the communication device can be done by one sector or multiple sectors depending on the degree of sector isolation. The same time and frequency resources may also be used on different sectors.
The combined cell extends the indoor and outdoor coverage of a single cell and entails several advantages. Coverage holes may be reduced by allowing multiple coverage areas within the same cell. The chance of receiving a communication device signal from selected sector carriers having the best coverage quality is increased. There is no need for inter-cell handover within a combined cell and uplink macro diversity is supported by the use of multiple antennas within the same (combined) cell.
From the above it is clear that the combined cell is an important feature of LTE. Uplink Frequency Selective Scheduling (UL FSS) is another important feature of LTE. In particular, UL FSS is a scheduling scheme which adopts frequency selective channel quality to prioritize communication devices and allocate resources. In UL FSS uplink, channel quality of the entire frequency spectrum is measured, and the part of the spectrum that has the best channel quality is allocated to the communication devices that have the highest priority. UL FSS provides increased throughput for cell edge users as well as increased cell throughput in a system using channel dependent scheduling.
In the deployment of combined cell, an evolved Node B (eNodeB or eNB) needs to keep track of which sector(s) a communication device should use. This sector selection is done by using an uplink reference signal called Sounding Reference Signal (SRS). Basically, the eNB orders periodic SRS transmissions across all sectors of the combined cell and by measuring, for a specific communication device, SRS received signal strength/channel quality on all sectors, the eNB can determine one or multiple sectors that the communication device should belong to.
UL FSS also uses SRS to obtain selective channel quality and hence schedule the communication device on the best possible frequency. Thus both sector selection and UL FSS rely on SRS measurements to function properly. However, the characteristics of SRS measurements required by these two features are very different. Sector selection based SRS measurement could work in a slow manner with less frequent updating, i.e. could use a SRS configuration with a rather long periodicity, and the communication device does not need to scan the whole frequency band. On the other hand, frequency selective scheduling and link adaptation requires fast updates and sub-band channel quality measurement, i.e. a SRS configuration with short periodicity should be used, and the communication device needs to scan the whole frequency band.
The UL FSS and combined cell are both important features and would both preferably be implemented simultaneously. In a combined cell configuration in which the UL FSS is also used, fast SRS are allocated to the communication devices to accommodate the need of the UL FSS for fast SRS resources. For sector selection purposes, this leads to SRS consuming more processing power than actually needed since more SRS measurements are received than needed, which in turn limits the system capacity. For UL FSS, given the currently supported number of active communication devices in a cell, this leads to the required SRS resource capacity exceeding the upper limit specified by 3GPP, making it unfeasible to use UL FSS in combined cell scenarios.
The conflict in the SRS configuration requirements has led to the fact that UL FSS cannot be used in combined cell deployment. The current solution has therefore resulted in that UL FSS is disabled in many cell configurations, and UL FSS and combined cell cannot be enabled simultaneously.